CEPHALOMETRICS- INSTRUMENTATION AND X – RAY GENERATION

PRINCIPLES

INTRODUCTION -

A scientific approach to the scrutiny of human craniofacial patterns was first

initiated by anthropologists and anatomists who recorded the various dimensions of

ancient dry skulls. The measurement of the dry skull from osteological landmarks called

craniometry, was then applied to living subjects so that a longitudinal growth study could

be undertaken. Since the measurements were taken through skin and soft tissue

coverage, their accuracy was questionable.

By the discovery of X-rays by Roentgen in 1895, a radiographic head image could

be measured in two dimensions, thereby making possible the accurate study of

craniofacial growth and development.

The credit of bringing the X – rays to the field of dentistry is given to C. Edmund

Kells. Soon after Roentgen announced his discovery in December 1895, Kells went to

work to make the capabilities of the X-ray available to the dental profession and thereby

forever changed the way dentistry would be practiced.

The measurement of head from the shadows of bony and soft tissue land marks on

the radiographic image became known as roentgenographic cephalometry.

(Krogman &Sassouni,1957)

HISTORY—

Cephlometrics like virtually all advances in healing arts is based on older methods.

Craniometrics was already being used to measure dried skulls, direct cephalometric

measurement was applied to external structures on the living and radiography was an

accepted clinical procedure. During the same period Pacini was also X-raying skulls in

Europe.

B. Holly .Broadbent merged those very different techniques to measure all three

dimensions of both internal and external structures of the heads of living subjects.

During 1920’s Broadbent refined the craniostat that was used to orient skulls for

measurement into a craniometer by the addition of metric scales. That proved to be the

1

first step in the evolution of the craniostat into a radiographic cephalostat. This direct

measuring instrument was later converted into a radiographic craniometer.

INTRODUCTION TO CEPHALOMETRICS—

Broadbent in 1931 introduced cephalometric radiography to overcome the

inappropriateness of earlier techniques in which the landmarks in the skull of the living

child had to be approached through the skin and soft tissue. Hofrath in Germany at the

same time developed his cephalometer independently.

At that time skull holders were used for craniometric studies . The first problem

Broadbent faced was to design and build a head holder along the lines of the skull holders

and second to find a means of recording precisely the craniometric as well as

cephalometric landmarks of the face and cranial base of the living head.

Keeping the Reserve craniostat as a basis, head holder was made and registering

of the internal landmarks of face and cranial base was made through perfection of a

roentgenographic technique that records these points accurately on the photographic film.

To test the accuracy of this method experiments were first made with skulls on a

specially constructed craniostat. The skulls were prepared by drilling a minute hole at

many of the internal and external cranial landmarks and inserting very small pieces of

lead that would register their exact position on the photographic film. Similar bits of lead

were placed on dental and facial points. The skulls were then clamped in the instrument

with the under surface of the upper side of the ear holes (external auditory meatus)

resting on the supports and the skull fixed in the Frankfort relation.

After the sites of the lead pieces were plotted in graphic projection in the sagittal

plane and their relationships defined by measurement, the skulls were x-rayed for the

lateral picture. Each skull was then rotated ninety degrees and measured in the frontal

plane, the graph made, and the frontal x-ray picture taken.

Superimposing the roentgenograms of the lateral and frontal projections on their

respective graphs, gave a measure of the technical precision and reliability of this

method.

2

Special Reserve Craniostat built for roentgenographic studies of skull

Two relations are necessary to produce two or more identical X- ray pictures of a skull.

1. Skull to the instrument .

2. Source of X-rays to the instrument.

Experimentally it was proven that most useful pictures were those made when the

path of the central ray coincided with the line joining the tops of the two ear supports and

the tube placed 5 feet or more from the middle of the craniostat.

The roentgenograms were measured with the aid of a Universal drafting machine

fitted with millimeter scales.

The head holder was designed on the working principles of the craniostat and built

for use in conjunction with the standard junior dental chairs, through the generosity of

Mrs. Chester. C. Bolton and her son Mr. Charles. B. Bolton.

The head holder was supported on a fixed base, above the child’s size dental chair

that has had the usual head rest removed. The chair does not come in contact with the

head holder but may be raised or lowered to permit comfortable adjustment of the child’s

head to the instrument.

The head rests on the upper most side of the calibrated ear rods, inserted into the ear

holes to allow centering of head. Then head is adjusted till the lowest point of the inferior

border of the left orbit is at the level of the top of the ear supports as indicated by the

orbital pointer.

3

There was a front attachment which supported the frontal cassette and it also carried

a rest for the root of the nose.

The Head Holder with cassette in its place for lateral roentgenogram.

The head could not be rotated on a vertical axis, so two X- ray tubes, like one for

frontal and one for lateral picture were used. The resulting pictures register precisely the

desired craniometric landmarks of the cranial base and face in three planes of space.

Subsequent pictures at certain ages, in children revealed areas of non-growth in the

cranial base. These areas were used to precisely relate the pictures and measure changes

in the other parts.

So the areas in the cranial base that have not changed, offer a more precise basis

for relating tracings and consequently a more accurate method of measuring growth

and development in the living head. Therefore when we have an unchanged base

common to two or more subsequent pictures of the same child, like the area including

Sella Turcica and Nasion of this series, we superimpose them on these landmarks.

This roentgenographic method has the added advantage of disclosing changes, not

only of the teeth that have erupted, but it clearly shows the rate and amount of growth and

path of eruption of the unerupted teeth. With the opportunity to record the structural

changes along with means of measuring increase in size, we have a morphological as

well as a quantitive study.

The lateral cephalometric radiograph (cephalogram) itself is the product of a two-

dimensional image of the skull in lateral view, enabling the relationship between teeth,

4

bone, soft tissue, and empty space to be scrutinized both horizontally and vertically . It

has influenced orthodontics in 3 major areas:

In morphological analysis, by evaluating the sagittal and vertical relationships of

dentition, facial skeleton, and soft tissue profile.

In growth analysis, by taking two or more cephalograms at different time

intervals and comparing the relative changes.

In treatment analysis, by evaluating alterations during and after therapy.

X – RAY GENERATION

The basic components of the equipment for producing a lateral cephalogram are-

X- ray apparatus

An image – receptor system

A cephalostat

X- Ray apparatus:

The basic apparatus for generating X- rays comprises of an X- ray tube, transformers,

filters, collimators, and a coolant system, all encased in the machine’s housing. The X-

ray tube is a high-vacuum tube that serves as a source of the x- rays. The 3 basic elements

that generate the x- rays are--

1. A cathode —a component of which is the filament that serves as source of

electrons.

2. An anode —(target) at which the beam of high speed electrons is directed.

3. Electrical power supply – through various circuits control tube performance.

Cathode – it is composed of 2 parts mainly

Filament

Focusing cup.

The filament, the source of electrons with in the X- ray tube, is a coil of Tungsten wire

about 0.2 cm in diameter and 1 cm or less in length. It is mounted on two strong stiff

wires that support it and carry the electric current. These 2 wires serve as a connection for

both high and low voltage electric sources. The filament is heated through incandescence

through a range of temperatures by varying the voltage (around 10 volts) across the

filament from a step down transformer in a low – voltage circuit.

5

The X –Ray tube with major component

The hot filament emits electrons at a rate proportional to its temperature by

thermoionic emission. The electrons lost by the filament form a cloud or space charge

about the filament and are replaced in the tungsten atoms from the negative side of the

high-voltage circuit, which is connected to one of the filament mounting wires.

Dental X-ray machine circuitry

In the figure; A is Filament step-down transformer ; B, filament current control (mA

switch; C, autotransformer; D, kVp selector dial (switch); E, high voltage transformer; F,

x ray timer (switch); G, tube voltage indicator (voltameter); H, tube current indicator

(ammeter); I, x- ray tube.

A milliamperage control, controls the flow of heating current through the filament,

thus thereby modulates the quantity of electrons that the filament emits, which inturn

controls the tube current and the number of X- ray photons subsequently produced.

6

Focusing cup: The filament is located in a focusing cup, a negatively charged concave

reflector of molybdenum. The focusing cup electrostatically focuses the electrons emitted

by the incandescent filament into a narrow beam directed at a small rectangular area on

the anode called the focal spot. Electrons move in this direction because of the strong

electrical field imposed between the cathode and anode.

Anode-

Anode is composed of a tungsten target and Copper stem. The purpose of the target in

an X- ray tube is to convert the kinetic energy of the electrons into X- ray photons.

Tungsten is usually selected as the target material because it represents an effective

compromise between the features of the ideal target material which are high atomic

number, high melting point and low vapor pressure at the high working

temperatures of an X-ray tube.

A target material with a high atomic number is best because it is more efficient for the

production of x- rays. High melting point is one of the major considerations in selection

of target material as 99% of kinetic energy of electrons is converted to heat. The low

vapor pressure of tungsten at high temperatures also precludes compromising the vaccum

in the tube at the high operating temperatures.

Thermal conductivity of tungsten is low, so it is embedded in a larger block of

copper, which dissipates heat. In addition, insulating oil may circulate between the

glass envelope and the protective tube housing. This type of anode is called stationary

anode.

Another method of dissipating the heat from a small focal spot is to use a rotating

anode. In this case the tungsten target is in the form of a beveled disc that rotates when

the tube is in operation. As a result of this arrangement the electrons strike successive

areas of the target as it rotates. This effectively widens the focal spot and distributes heat

over this expanded area. Such rotating anodes are not used in conventional dental x- ray

machines but may be used in cephalometric units and in medical x- ray machines.

Radiographic image quality is dependent in part on the geometry of the target. The

sharpness of the radiographic image increases as the size of the radiation source , the

focal spot, decreases. To take advantage of the benefits of a smaller focal spot, yet

7

effectively distribute the bombarding electrons over the greater surface of a larger target,

the target is placed at an angle with respect to the electron beam in the tube.

Target placed obliquely to the central ray.

The projection of the focal spot perpendicular to the electron beam (the effective focal

spot) will be smaller than the actual size of the focal spot. The use of an anode with the

target angulated such that the effective focal spot is smaller than actual focal spot size is

referred to as the Benson line focus principle .

Power supply-

The primary functions of the power supply is to provide

1. A current to heat the X –ray tube filament by use of a step-down trasformer

2. A potential difference between anode and the cathode.

The filament step down transformer reduces the voltage of the incoming alternating

current to less than 10 volts and its operation is regulated by the filament current control

switch, which adjusts the current flow through the low-voltage circuit and thus the

filament. This in turn regulates the heating of the filament and thus the quantity of the

electrons emitted. The electrons emitted by the filament travel to the anode and

constitute the tube current.

The output of autotransformer is regulated by Kvp selector dial, which select

varying voltages and it is applied to the primary of the high voltage transformer, which

controls the voltage between the anode and cathode of the X- ray tube. The high voltage

transformer provides the high voltage required by the x- ray tube to accelerate the

8

electrons and generate x rays. It accomplishes this by boosting the voltage of the

incoming line current to 60 to 100 Kvp.

Since the line current is an alternating current (60 cycles/ sec), the polarity of the X

– ray tube will alternate at the same frequency. When the electrons strike the focal spot

of the target, some of their energy converts to x- ray photons. X-rays are produced at the

target with greatest efficiency when the voltage applied across the tube is high. Thus the

intensity of x-ray pulses will tend to be sharply peaked at the center of each cycle.

During the following half (or negative half) of the cycle, the polarity of the AC

reverses and the filament becomes positive and the target negative. At these times

the electrons stay in the vicinity of the filament and do not flow across the gap

between the two elements of the tube. This voltage is called inverse voltage or reverse

bias. No x rays are generated during this half of the voltage cycle. Thus, when an x- ray

tube is powered with 60-cycle alternating current, 60 pulses of x- rays are generated each

second, each having duration of 1/20 second. This type of power supply circuitry, where

alternating high voltage is applied directly across the tube, limits X- ray production to

half of AC cycle is said to be self or half wave rectified.

A tube energized with a self-rectifying power supply must not be operated for

extended periods or the temperature of the target may reach the point of electronic

emission. If the target gets that hot, there is the possibility that during the negative half-

cycle the inverse voltage will drive electrons to the filament, causing it to overheat and

melt. The glass envelope may also be damaged if the electrons are driven in the wrong

direction by the reverse bias on the tube.

Some units have half wave tube rectification where the inverse voltage is

prevented from being applied across the tube during the negative half of the cycle.

Full wave rectification units are also used in some machines, that allows both

positive and negative phases to be utilized for X- ray production.

Timer-

The timer completes the circuit with the high-voltage transformer. This controls the time

that the high voltage is applied to the tube and thus the time during which tube current

flows and x rays are produced. Before the high voltage is applied across the tube,

9

however, the filament must be at the proper operating temperature to assure an adequate

rate of electron emission. It is not practical to subject the filament to prolonged heating

at normal operating current.

Production of X- rays

The kinetic energy of electrons in the tube current is converted into X- ray photons

at the focal spot of an X-ray tube by two mechanisms

1. Bremsstrahlung radiation

2. Characteristic radiation

Bremsstrahlung radiation

Bremsstrahlung interaction,the primary source of an X-ray photons from an X- ray

tube , is produced by the sudden stopping or braking of the high speed electrons at the

target . The electrons are accelerated by the high voltage applied across the gap between

the filament and the target of the x – ray tube. When the electrons interact with the

electrostatic field of target nuclei of collide with nuclei, their direction of travel is altered.

This process of rapidly decelerating the high speed electron is called inelastic collision

and gives rise to Bremsstrahlung or braking radiation. This deceleration causes them to

lose some kinetic energy, which is given off in the form of photons of electromagnetic

radiation with an energy equal to that lost by the deflected electrons.

Bremsstrahlung interaction generates photons having a continuous spectrum of

energy. The reasons for this continuous spectrum are as follows:

1. The continuously varying voltage difference between the target and filament,

which is characteristic of half-wave rectification, cause the electrons striking the

target to have varying levels of kinetic energy.

2. Most electrons participate in many interactions before all their kinetic energy is

expended. As a consequence, an electron will carry differing amounts of energy at

the time of each interaction with tungsten atom that results in the generation of an

x – ray photon.

3. The bombarding electrons pass at varying distances around tungsten nuclei and

are thus deflected to varying extents. As a result, they give up varying amounts of

energy in the form of Bremsstrahlung photons.

10

Characteristic radiation

It occurs when a bombarding electron displaces an electron from a shell of the

target atom, thereby ionizing the atom. So an electron from an outer shell (higher energy

level) occupies this vacancy, and gives off a photon, with an energy equivalent to the

difference in the two orbital binding energies. The energies of characteristic photons are a

function of the energy levels of various electron orbital levels and hence are characteristic

of the target atomic composition. Characteristic radiation is only a minor source of

radiation from an x- ray tube.

Image receptor system:

An image receptor system records the final product of X-rays after they pass through

the subject. The extraoral projection,like the lateral cephalometric technique, requires a

complex image receptor system that consists of an extraoral film, intensifying screens, a

cassette, a grid, and a soft- tissue shield.,

Films :

The X- ray image formed when X – rays pass through the patients head is recorded

by a film- screen combination enclosed in a cassette. Film size is usually 8X10” or

10X12” for some other purposes.

Basic components of the x-rays film are an emulsion of silver halide crystals

suspended in a gelatin framework and a transparent blue- tinted cellulose acetate that

serves as the base.

When the silver halide crystals are exposed to the radiation, they are converted to

the metallic silver image. This is converted into a visible and permanent image after film

processing. The amount of metallic silver deposited in the emulsion determines film

density, whereas the grain size of the silver halide determines film sensitivity and

definition.

Intensifying screens:

They are used in pairs together with a screen film to reduce the patient’s exposure

dose and increase image contrast by intensifying the photographic effect of x- radiation.

11

X – rays are more readily absorbed by heavier atoms of materials. So the

intensifying screen is made of crystals of a high atomic no. material that absorbs X – rays

efficiently and convert it into light energy (fluoresce). This is absorbed by the light

sensitive radiograph, which is then processed to produce the radiograph.

The most commonly used screens are made of calcium tungstate,barium lead

sulphate crystals and are sensitive to X- rays generated by conventional dental X – ray

machines (60-90 Kvp).

More efficient rare earth intensifying screens using terbium activated gandolinium

oxysulfide and thalium activated lanthanum oxybromide can be used with special X- ray

film sensitive to the green spectral emission of rare earth screens. Calcium tungstate

screens emit blue light.

Films and screens may have fast, medium or slow speed depending on the crystal

size and thickness. High speed films and screens produce les detail and less sharp images

in radiographs.

Cassettes

They are light tight boxes used to hold the screens and film in intimate contact.

Cassettes contain two screens with a double emulsion film sandwitched between the

screens. Cassettes may be equipped with front and back screens with different speeds.

With the high speed screen in the back if a single emulsion film is used, only one screen

is needed in the cassette, but it may require more exposure.

Films should be handled carefully, by keeping them away from excessive

temperature or humidity. Rapid removal of the film from the cassette can produce

electrical discharges that can cause artifacts in the radiograph.

Grids

Scattered or secondary radiation causes the film to fog. It can be prevented by

placing a grid between the patient and the film. Grid consists of alternating strips of

radiopaque and radioluscent material. While the lead strips block some of the X- rays

coming from the tube, they effectively block the scattered rays that are traveling in

directions oblique to the X-ray beam.

12

The grid lines that appear in a radiograph can be avoided by moving the grid in a

direction that is at a right angle to the line during film exposure. Such a grid is called a

Bucky grid or a potter- Bucky diaphragm. Non –moving grid is called a stationary grid.

CEPHALOMETRIC RADIOGRAPHY--

Cephalometric radiographs can be made with conventional dental X- ray machine

used for making intra oral radiographs. These machines usually use a self rectified X- ray

tube with a stationary anode. 10 – 15 mA and 70 Kvp (peak kilovoltage). For

cephalometric purpose, the tube head of this type of machine is fixed to a stationary

device to direct the X- ray beam in a fixed position relative to the patient and film. With

medium speed film and screens, the exposure time is approximately 0.6 to 1.2 seconds.

X-ray generators capable of producing x- ray beams with great intensities of x-

radiation are available. These machines use either a 100 m A current or 100 Kvp a

rectified electric current to the tube and/or a rotating anode in the x- ray tube.

These facilitate the use of short exposure time ( in the region of 1/60 th of a second),

which can reduce motion unsharpness in the radiographic image.

Panoramic X – ray machines with the capability of aligning the tubes for

cephalometric radiography is also available.

PATIENT POSITIONING --

The patient is positioned differently on the X- ray beam for lateral, PA and oblique

views of the skull. Patient is in an upright position, either sitting or standing, with the X –

ray generator and film at a fixed height and a system for raising and lowering the patient

by using a motorized chair.

A cephalostat or head holder is used to stabilize the patient in a fixed position in

the X- ray beam. It basically consists of two ear rods that move simultaneously or

individually along the path of the central ray. The device holds the patient steady with the

central ray in the transmeatal axis. Many adjustments have been calibrated to standardize

patient position and caphalostats with measurement capabilities are called cephalometers.

Standardizing the Frankfort horizontal plane is accomplished on a cephalometer

with an orbital pointer. The pointer consists of a vertically adjustable horizontal rod that

13

is positioned at the patients’ orbitale. Another method is the use of a forehead positioner

located at Nasion. Some operators prefer to have the patients oriented to natural head

position, it is accomplished by asking the patient to look directly into the mirror image of

their own eyes.

To orient the patients mid saggital plane in a vertical position, a vertical line may

be placed on the mirror where the center of the cephalometric image is located as seen

from patient position.

Although the patient to source distance is standardized as 5 feet, the patient to film

distance may vary, thus varying the magnification. To calculate this, a radiopaque scale is

kept in the midsaggital plane and its magnification is measured.

Most cephalometers can be rotated through 360° on the vertical axis in contrast to

Broadbent – Bolton Roentgenographic Cephalometer, where two X- ray tube head

film holder systems were used at right angles to each other.

The orientation of the Frankfort plane around the transmeatal axis is important

because the superposition of different parts of the skull upon each other can occur with

different Frankfort plane positions it should also be standardized to make reliable

comparisions.

In dental cephalometric radiography, position of patients’ mandible is not fixed in

the cephalometer. Cephalometric radiographs are made with patients’ teeth in occlusion,

it can also be made in rest position or wide open position if desired.

THE THIRD DIMENSION

Clinical orthodontics is yet to fully utilize Broadbent’s contributions. He gave us a 3

dimensional analysis. But still in most clinical practices lateral roentgenographic view is

utilized. The lateral view is to work with and the patient is also much more recognizable

than in frontal (PA) view, especially with soft tissue enhancement. But it is not enough.

We treat in 3 dimensions, and the width dimensions that are visualized on the frontal

view are crucial in many cases. In these days of increasing awareness of the contributions

of muscular and esthetic function, we can no more afford to continue to close our eyes to

the information in the frontal view.

14

POSTEROANTERIOR (FRONTAL) CEPHALOMETRY

Malocclusion and dentofacial deformities constitute three dimensional conditions or

pathologies. Although all orthodontic patients deserve an equally comprehensive three-

dimensional diagnostic examination, assessment of posteroanterior cepalometric views

are of particular importance in cases of :

1. dentoalveolar and facial asymmetries

2. dental and skeletal crossbites

3. functional mandibular displacements.

The same equipment that is used for lateral cephalomeric projections is utilized, i.e.

a head holder or cephalostat, an x ray source, and a cassette holder containing the film.

The initial unit described by Broadbent consisted of a set up in which two x ray

sources with two cassettes were simultaneously used, so that lateral and frontal

cephlograms were taken at the same time.In this technique, the patient was placed with

the Frankfort horizontal plane parallel to the floor. The x ray source exposing the

cassette for the poteranterior cephalogram was 5 feet away from the earpost axis, behind

the patient, and the central x ray beam passed at the level of the Frankfort horizontal

plane and at a 90 degree angle to the beam of the lateral cephalogram. Although precise

three dimensional evaluations are possible using this technique, it has now been almost

abandoned since it requires a rather large equipment with two x ray sources.

Modern equipment uses one x ray source. A cephalostat that can rotated 90° is

used so that the patient can be repositioned, without any alteration in the Frankfort

horizontal relationship of the head to the floor, for taking the P-A cephalogram.

Maintaining this identical horizontal orientation from lateral to postero-anterior

projection is critical when comparative measures are made from one to the other.

(Moyers et al, 1988)

Natural head position as mentioned earlier, is a standardized orientation of the head,

which is readily assumed by focusing on a distant point at eye level. In using the natural

head position for poteroanterior cephalometric registrations, some practical problems are

encountered. The patients head is facing the cassette, which makes it difficult for the

patient to look into a mirror to register natural head position.

(Solow and Tallgren, 1971)

15

Furthermore, space problems make it impossible to place a nosepiece in front of

nasion to establish support in a vertical plane.

For better evaluation of patients with craniofacial anomalies that require special

attention to the upper face, the patient head should be positioned with the tip of the nose

and forehead lightly touching the cassette holder.

In cases of suspected significant mandibular displacement, the PA cephalogram

should be taken with mouth of the patient slightly opened in order to differentiate

between functional mandibular displacement and dentoskeletal facial asymmetry.

(Faber, 1985)

As far as exposure conditions and considerations are considered, more exposure is

necessary for PA cephalograms than for lateral views.

(Enlow, 1982)

QUALITY OF THE RADIOGRAPHIC CEPHALOMETRIC IMAGE

Image quality is a major factor influencing the accuracy of cephalometric analysis.

An acceptable diagnostic radiograph is considered in the light of two groups of

characteristics:

Visual characteristics

Geometric characteristics.

Visual characteristics

The visual characteristics – density and contrast – are those that relate to the

ability of the image to demonstrate optimum detail within anatomical structures and to

differentiate between them by means of relative transparency.

Density—

Density is the degree of blackness of the image when it is viewed in front of an

illuminator or view box. The radiographic density is calculated from the common

logarithm of the ratio of the intensity of the light beam of the illuminator striking the

image to the intensity of the light transmitted through the film.

16

As the x ray image is formed as a result of processing in which the silver halide

crystals in the emulsion of the film being exposed to the x rays are converted to the

metallic silver, the two main factors that control the radiographic density are-

1. The exposure technique

2. The processing procedure.

Exposure technique --

The exposure factors related to image density are-

tube voltage (kilovoltage peak, kVp)

tube current (milliamperage, mA)

exposure time (second,S)

focus-film distance (D)

The relationship of image density and these factors is expressed as an equation:

Density= (kVp X mA X S)/D

Processing procedure --

Film processing consists of developing , rinsing, washing , drying, and mounting

the exposed film. An invisible image, produced when the silver halide crystals are

exposed to the x rays is altered to a visible and permanent image of the film by chemical

solutions. The image density is directly proportional to temperature of the developing

solution and developing time.

The size of silver halide crystals in the film emulsion determines the film speed.

A film with large grain size (high-speed film) produces greater density than a film with

small grain size.

Contrast-

Contrast is the difference in densities between adjacent areas on the radiographic image.

Factors controlling the radiographic contrast are:

Tube voltage - the kilovoltage peak has the most effect on radiographic contrast.

When the kilovoltage peak is low , the contrast of the film is high, and the film

has short-scale contrast. On the other hand, if the kilovoltage peak is high , the

contrast of the film is low , and the film has long-scale contrast.

17

Secondary radiation or scatter radiation - the secondary radiation caused by low

energy x ray beams decreases the contrast by producing film fog. The amount of

secondary radiation is directly proportional to the cross-sectional area, thickness

and density of the exposed tissues. Various soft tissue shield has been

incorporated into the cephalometric system to remove secondary radiation,

including an aluminum filter, lead diaphragm and grid.

Subject contrast – this refers to the nature and properties of the subject, such as

thickness, density, and atomic number.

Processing procedure – the temperature of the developing solution affects image

contrast. The higher the temperature the greater the contrast.

Density and contrast are the image characteristics that are usually affected when

the kilovoltage peak is altered. However , only the radiographic density can be altered

without changing the contrast when the kilovoltage peak is constant and the

milliamperage –second is altered.

Geometric characteristics --

The geometric characteristics are-

1. image unsharpness

2. image magnification

3. shape distortion

Radiographic image produced by divergent beam

18

X rays by their nature are divergent beams radiated in all directions. Consequently, when

they penetrate through a 3 dimensional object such as a skull, there is always some

unsharpness and magnification of the image, and some distortion of the shape of the

object being imaged.

The focal spot from which the x-rays originate, although small , has a finite area,

and every point on this area acts as an individual focal spot for the origination of x ray

photons. Therefore, most of the x rays emitted from the focal spot are actually producing

a shadow of the object(the Umbra).

Image unsharpness--

Image unsharpness is classified into three types according to etiology, namely:

geometric, motion and material.

Factors influencing the size of the penumbra.

19

Penumbra size decreases if the focal spot size decreases(B), the focus – film distance

increases (C), or the focus – film distance is increased while object-film distance is

decreased(D).

Geometric unsharpness is the fuzzy outline in a radiographic image caused by the

penumbra. Factors that influence the geometric unsharpness are size of the focal spot,

focus-film distance, and object-film distance. In order to decrease the size of the

penumbra, the focal spot size and the object-film distance should be decreased and the

focus-film distance increased.

Geometric unsharpness is defined by the following equation-

Geometric unsharpness = (focal spot size X object-film distance)/focus-film distance.

Motion unsharpness is caused by movement of the patients’ head and movement of

the tube and film.

Material unsharpness is related to two factors.

1. it is directly proportional to the grain size of the silver halide crystals in the

emulsion.

2. it is related to the intensifying screens, which , although they can minimize x ray

dose to the patient, also result in unsharpness that is related to the size of the

phosphorescent crystals, the thickness of the fluorescent layer, and the film-screen

contact.

Image magnification

It is the enlargement of the actual size of the object. Factors influencing image

magnification are the same factors as those that influence geometric unsharpness (i.e. the

grain size of the silver halide crystals in the emulsion, and various features of the

intensifying screens).

Shape distortion

It results in an image that does not correspond proportionally to the subject. In the

case of a skull, which is three – dimensional object, the distortion usually occurs as a

result of improper orientation of the patient’s head in the cephalostat or improper

20

alignment of the film and central ray. This kind of distortion can be minimized by placing

the film parallel to the midsagittal plane of the head and projecting the central ray

perpendicularly to the film and the midsagittal plane.

FACTORS AFFECTING THE QUALITY OF THE IMAGE

Quality of the image is controlled by the manufacturer of the X- ray equipment and

by the operator.

Manufacturer provides pre programmed exposure factors consisting of mA, Kvp

and exposure time (S), which enable image density and contrast to be controlled when

object density and thickness are varied. The variations in the exposure factors depends on

the type of X- ray machine , target –film distance, the film-screen combination and the

grid chosen.

Tube current : theoretically there is a linear relationship between mA and tube output.

Thus the quantity of radiation produced by an x- ray tube (i.e. the number of photons that

reach the patient and film) is directly related to the tube current and the time the tube is

operated.

Spectrum of photon energies showing effect of tube current

The quantity of radiation produced is expressed as the product of time and tube

current. The quantity of radiation will remain constant regardless of how mA and time are

changed if their product remains constant.

Exposure time: is the commonest factor to change, since altering it has greatest effect

on image density.

21

Spectrum of photon energies showing effect of exposure time

When exposure time is doubled, the number of photons generated is doubled but

the range of photon energies is unchanged. Thus the effect of changing time is simply to

control the quantity of the exposure (the number of photons generated).

Tube voltage: Altering Kvp not only affects image contrast but also the exposure time.

When kVp is increased the spectrum of energy range , as well as, the number of photons

produced at each energy value, and the average energy of the beam of photons will be

increased. Thus as the kVp is increased there is an increase in the energy of each electron

has when it strikes the target. This results in an increased efficiency of conversion of

electron energy into x- ray photons, and thus in an increase in the

1. number of photons generated,

2. mean energy of the photons

3. maximum energy of the photons.

Spectrum of photon energies showing effect of tube voltage

This results from greater efficiency in the production of bremsstrahlung photons

when increased numbers of higher energy electrons interact with the target.

22

Image density and contrast can also be affected by film processing. When using an

automatic film processor, these factors are controlled by the temperature of the developer

and developing time.

Optimum temperature being 68°F and time 5 minutes.

Image sharpness and magnification are controlled by manufacturer and operator.

Manufacturer provides the most effective focal spot size, target film distance ,collimation

and filtration measures to produce maximum X- ray beams with best size and shape.

Filtration-

An x ray beam consists of a spectrum of x ray photon of different energies, but only

photons with sufficient energy to penetrate anatomic structures are useful for diagnostic

radiology. Those that are of low penetrating (long wavelength ) contribute to patient

exposure but not to the information on the film. Consequently , in the interest of patient

safety , it is necessary to increase the mean energy of the x ray beam by removing the less

penetrating photons. This can be accomplished by placing an aluminum filter in the path

of the beam.

The aluminum filter removes many of the lower energy photons with little affect of

those that are able to penetrate the patient and reach the film.

The inherent filtration of the tube and its housing consists of the materials that x ray

photons encounter as they travel from the focal spot on the target to form the usable beam

outside the tube enclosure. These materials include the glass wall of the x ray tube, the

insulating oil that surrounds many dental tubes, and the barrier material that prevents the

oil from escaping through the x ray machines ranges from the equivalent of 0.5 to 2 mm

of aluminum.

Total filtration is the sum of the inherent filtration plus any added external filtration

supplied in the form of aluminum disks placed over the port in the head of the x ray

machine.

Collimation-

Collimation means to shape an x ray beam, usually by the use of metallic barrier

with an aperture in the middle collimation reduce the size of the x ray beam and thus the

23

volume of irradiated tissue within the patient from which the scattered photons originate.

Collimation thereby reduces patient exposure and increases film quality.

Diaphragm, tubular, and rectangular collimators are useful in dentistry. The diaphragm

collimator is a thick plate of radiopaque material (usually lead) with an aperture or

opening in it that is usually placed over the port in the x ray head through which the x ray

beam emerges.

Inverse square law--

The intensity of an x ray beam at a given point is dependent on the distance of the

measuring device from the focal spot. For a given beam, the intensity is inversely

proportional to the square of the distance from the source. The reason for this decrease in

intensity is that the x ray beam spreads out as it moves from the source.

Relation between the intensity of radiation and focus-film distance

Changing the distance between the x ray tube and the patient thus has a marked

effect on beam intensity, such a change will require a corresponding modification of the

kVp or mAs if the exposure of the film is to be kept constant.

24

USES—

Cephalometrics is not only a research tool. It is also useful for diagnosis, treatment

planning, prognosis, surveying the results of treatment, and for following or even

predicting growth. It is not confined to orthodontics, but can give valuable information to

the oral surgeon, plastic surgeon, prosthodontist, pedodontist, and speech pathologist.

Common clinical applications of Cephalometrics are:

to evaluate dentofacial propotion and clarify the anatomic basis for a

malocclusion

by means of cephalograms taken before, during and after orthodontic treatment it

is possible to recognize and evaluate changes brought about by the treatment.

To predict changes that should occur in the future for a patient.

Although cephalograms are not taken as a screen for pathology, but there is a

possibility of observing pathological changes on the cephalogram.

LIMITATIONS—

They give two-dimensional image of a three-dimensional object.

There can be errors while developing cephalograms which can limit measuring

accuracy to 0.5 mm:

Movement of the subject,

Optical blurring (depends on the size of the focal spot),

Grain size of film and intensifying screens.

This is why it is important to keep the subject-film distance as nearly constant as

possible. This is particularly true for linear measurements which are going to be enlarged

about 10 percent. Distortion enters in when landmarks are used which are not in the

midsagittal plane. The points nearest the film will be enlarged the least.

ADVANCEMENT IN THE INSTRUMENTATION

Bjork in 1968 designed an X- ray cephalostat research unit with a built in 5 inch

image intensifier that enabled the position of the patients’ head to be monitored on a TV

screen. It also allowed cephalometric X-ray examination of oral function on the TV

screen, which could also be recorded.

25

In 1988, Solow and Kreiborg, developed a multiprojection cephalometer, which

featured improved control of head position and digital exposure control. It uses laser

beams for head positioning.

Units have also been developed for roentgenocephalometric registeration of

infants.

Digital imaging in dentistry is a rapidly changing field. Within the last five years

new devices and computers systems have been introduced to record X-ray images and to

manipulate those images using a variety of image processing operations.

Combining radiology with telecommunications has produced teleradiography, the

transmission of radiographic images over telephone lines. In medicine, sharing images

with a colleague to whom you have referred a patient, or consulting with a colleague at a

distant facility is now feasible. Applications in dentistry may become commonplace in

the future.

CONCLUSION --

Cephalometric radiographic techniques has advanced much on the solid basis put up by

Hofrath and Broadbent.

The cephalometric radiography has influenced orthodontics in 3 major ways

1. in morphological analysis, by evaluating. The saggital and vertical relationships

of dentition, facial skeleton and tissue profile.

2. in growth analysis- by comparing cephalograms taken at different time intervals.

3. in treatment analysis- by evaluating alterations during and after therapy.

But due to increasing awareness among patients about esthetic needs , functional

requirements has made it imperative to look seriously for getting 3 dimensional view and

frontal radiographic views which were largely being ignored till now, for providing better

patients satisfaction.

26

REFERENCES:

1. Orthodontic Cephalometer- Athanasios E. Athanasiou

2. Oral Radiology – Principles and Interpretation—Goaz &White.

3. Broadbent . B . – Angle Orthodontics 1, 45, 1931.

4. Sassouni. V. – AJO-41,735,1955.

5. Hofrath – Fortscher Orathhodontics 1:232-48, 1931.

6. Sollow B, Tallgren A.—Acta. Odontol. Scand, 597-607, 1971.

7. Pacini AJ, -- J. Radiology, 3:230-238, 1922.

8. Bjork A.—Am.J. Phys. Anthropology. 29:243-254, 1968.

27